ExplorationSecond wave of Brazil pre-salt finds poses tougher challengeSpate of disappointing results on high-cost wells raises questions about prolific Brazilian play that has yielded 28 billion barrels of reserves in a decadeProbing: Seadrill drillship West Tellus is currently drilling for Petrobras and ExxonMobil on an unproven area of the pre-salt playPhoto: SEADRILLGareth ChetwyndLondon